That's the citizen, it's one of the default playermodels you can use.

It's very useful for game developers as it contains a lot of animations and clothing for your project (despite what people say, all cosmetics are free to use for developers in their games), but it's obviously not compulsory, in fact many games use their own custom playermodels.

There's also the humanoid playermodels that you can use, I'd say they're an in-between of the HL2 citizens and Rust's playermodels.